The present invention relates to a method for the preparation of an alloy of nickel and titanium or, more particularly, to a method for the preparation of an alloy of nickel and titanium from powders of the respective metals by heating only at a temperature much lower than the eutectic point of an alloy with the same proportion of the two metals.
As is known, alloys of nickel and titanium are important corrosion-resistant materials and widely used in many applications. In addition thereto, a novel application has been developed in recent years for the alloys of nickel and titanium as a shape-memory alloy highlighted in the modern technology with rapidly increasing demand for the alloys.
Generally speaking, alloys of two kinds or more of different metals are prepared by the method of fusion in which the respective metals are melted by heating at a temperature higher than the melting point of the metal and the molten metals are uniformly mixed together or, alternatively, by the method of diffusion in which solid metals are kept in intimate contact at a temperature somewhat lower than the melting point for a sufficiently long time so that the atoms of the different metals diffuse each into the other to form an alloy.
When an alloy of nickel and titanium is to be prepared by the conventional fusion method, for example, the respective metals must be melted by heating at a temperature of 1500.degree. C. or higher and kept at the temperature for a long time. When the diffusion method is undertaken for the preparation of an alloy of nickel and titanium, the blend compact of the metal powders must be sintered at 1000.degree. C. or higher for a length of time still longer than in the fusion method. The alloying procedure at such high temperatures is unavoidably accompanied by a problem that the alloy is more or less contaminated by the oxides and carbides so that the purity of the alloy is usually not high enough unless the alloying procedure is undertaken under utmost care with great troublesomeness in addition to the expensiveness of the apparatus used in carrying out the high-temperature process. The process temperature can be decreased by increasing the proportion of nickel in the alloy to decrease the transformation point. The application field of such a high-nickel alloy, however, is limited due to the increase in the hardness of the alloy to cause difficulties in working or, in particular, in the fabrication of springs from a wire of a large diameter.